This invention relates to ferroelectric liquid crystal cells, and is particularly concerned with the quality of the bistability operation exhibited by such cells.
A paper by Y Sato et al entitled `High Quality Ferroelectric Liquid Crystal Display with Quasi-Bookshelf Layer Structure`, Japanese Journal of Applied Physics Vol. 28, No. 3, 1989 pp L 483-L 486, to which attention is directed, describes how the memory properties of a ferroelectric cell with one form of rubbed alignment can be significantly improved by pre-treatment of the cell with a relatively large amplitude low frequency field, but that in the case of a similar rubbed alignment cell constructed with a different material for the rubbed alignment layers the pre-treatment produced no apparent change in memory properties. The authors explain that the bookshelf layer structure (also sometimes known as bookstack layer structure) that Clark and Lagerwall originally proposed is now known to be difficult to achieve, and that a typical `uniform mode` ferroelectric cell relaxes after removal of a switching stimulus in a way that significantly reduces the contrast between the two switched states. The authors then explain that, in the case of a cell with rubbed N-methyl aminopropyl triethoxysilane alignment layers, the pre-treatment has the effect of changing the texture of the relaxed state, removing zigzag defects and replacing them with long slender domains, and that thereafter ordinary switching of the cell produced substantially fully switched states that show no noticeable relaxation upon removal of the switching stimulus. In the case of a cell with rubbed polyimide alignment layers, the authors report that the pre-treatment produced a similar change in appearance, but that this was not attended by an improvement in memory properties; in ordinary switching of the cell it relaxed after removal of the stimulus in substantially the same way as it used to relax before pre-treatment.
A somewhat similar effect is also reported in UK Patent Application GB 2 183 054A in which a similar treatment is reported to have been applied to various uniform mode ferroelectric cells, all with rubbed polymer alignment layers, but using different polymers in different cells. In each case, including the example using polyimide alignment layers, memory properties were reported as being improved by the substantial elimination of relaxation effects, though the effect of the treatment was said to be `of finite duration` and thus typically requiring `refreshing after standing for several days`.